program Project1;

{$APPTYPE CONSOLE}

uses
  SysUtils;

var
  n, i, summ : integer;
  mass : array of integer;


begin
  readln(n);
  setlength(mass, n+1);
  for i:=0 to n do
    mass[i]:=i;
  summ:=0;
  if n mod 2=0 then
    begin
      for i:=0 to n do
        begin
          if (i mod 2=0) then
          summ:=summ+i;
        end;
    end
  else 
    begin
      summ:=0;
      for i:=3 to n do
        begin
        if (i mod 2=1) then
        summ:=summ+i;
        end;
    end;
  writeln(summ);
  readln;

end.
 